I noticed that my cable modem has automatically switched to a new IP address (note that this is not the gateway box's address which remains the same throughout my connection contract so far). This is a remotely administrable cable-modem (Cornerstone CM200) that the ISP can configure settings at their site. I found that the cable-modem remains on the same subnet, fortunately, so a simple change of the IPF rules is necessary.

If your provider changes the modem IP (if you notice that you are blocking new DHCP configure requests - UDP/67->UDP/68), then you might also have to change the rule a bit. It's straight-forward. Basically, instead of having the IP address of the cable-modem be 203.123.123.12/32, I've changed it to be 203.123.1.1/16. (Yes a /16 because I noticed this significant change in the modem's IP address). So the rule looks like: